1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side light type including a light guide plate, wherein a light source is disposed at a light entrance end surface of the light guide plate and light for illumination is emitted from a light emitting portion of the light guide plate in a spread manner, and relates more particularly to a spread illuminating apparatus including a light guide plate having a slope portion which is disposed toward the light entrance end surface and in which a thickness decreases gradually with an increase of distance from the light entrance end surface.
2. Description of the Related Art
A spread illuminating apparatus of side light type (back light), in which a plurality of LEDs (light emitting diodes) having a small dimension and excellent environmental compatibility are disposed along a light entrance end surface of a light guide plate, is widely used as an illuminating means for a liquid crystal display panel particularly in the field of small handheld terminals such as mobile phones. Recently, in order to meet the ever increasing demand for further reducing the thickness of small handheld terminals, a light guide plate is extensively employed that includes a slope portion disposed between the light entrance end surface and a light emitting portion from which light is emitted in a spread manner, wherein the slope portion has a thickness decreasing gradually with an increase of distance from the light entrance end surface (refer to, for example, Japanese Patent Application Laid-Open No. 2007-287550).
When a slope portion is formed at a portion of a light guide plate located close to a light entrance end surface as described above, the thickness of a light emitting portion of the light guide plate can be reduced regardless of the thickness of an LED. However, when the slope portion is disposed close to the light entrance end surface of the light guide plate as described above, some of light which enters the light guide plate from the light entrance end surface is easily allowed to leak from a slope surface of the slope portion. Also, since the light is reflected at the slope surface with a large directional change and thus is incident on an opposite surface at a small angle, much light is caused to exit from a part of the light emitting portion located close to the slope portion. Consequently, the utilization efficiency of light emitted from the LED is reduced and at the same time the brightness uniformity of illumination light emitted from the light emitting portion is deteriorated. Under the circumstances, a spread illuminating apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-287550 includes a light guide plate in which an angle (refer to θ in FIG. 7B) formed between a light emitting surface and a slope portion is set at or below a prescribed value to thereby solve the problems.
Recently, in the effort to achieve a maximum liquid crystal display area, a spread illuminating apparatus is increasingly requested to have not only a lower profile but also a reduced non light emitting area (narrower picture frame) formed around a light emitting area. In order to satisfy the requests, since the slope portion cannot be utilized as a light emitting area for the spread illuminating apparatus, the dimension of the slope portion measured along the direction of light progression (length of the slope portion) must be minimized without allowing the amount of light leakage to increase. That is to say, the angle formed between the light emitting surface and the slope surface must be set at the largest possible value, which conflicts with the above disclosure that the angle formed between the light emitting surface and the slope surface is set at or below a prescribed value in order to improve the light utilization efficiency and also enhance the brightness uniformity of illumination light. Accordingly, it is necessary to develop a design concept that enables the non light emitting area to be minimized without allowing light loss to increase.
Also, the following problem is caused when the slope portion is provided toward the light entrance end surface of the light guide plate. Usually, LEDs are mounted on a strip-shaped circuit substrate and disposed to oppose the light entrance end surface of the light guide plate such that a part of the circuit substrate located forward of the mounted LED is fixedly attached to a part of the light emitting surface of the light guide plate located toward the light entrance end surface whereby the LEDs are fixed in position with respect to the light guide plate. This causes no problem when a slope portion is not formed toward the light entrance end surface of the light guide plate. Specifically, referring to FIG. 7A showing a light guide plate 103 having no slope portion provided toward a light entrance end surface 103a, when a circuit substrate 102 is fixedly attached to a light emitting surface 103b of the light guide plate 103, a light emitting face 101a of an LED 101 is set parallel to the light entrance end surface 103a of the light guide plate 103.
On the other hand, in the case where a light guide plate 113 includes an slope portion provided toward a light entrance end surface 113a as shown in FIG. 7B, when the above mentioned circuit substrate 102 is fixedly attached to a slope surface 113b of the slope portion of the light guide plate 113, the light emitting face 101a of the LED 101 cannot be set parallel to the light entrance end surface 113a of the light guide plate 113, because the light guide plate 113 is structured such that the light entrance end surface 113a is not orthogonal to the slope surface 113b. As a result, part (indicated by a dashed line with an arrowhead in FIG. 7B) of light emitted from the light emitting face 101a of the LED 101 leaks outside instead of being incident on the light entrance end surface 113a of the light guide plate 113, which lowers the brightness of illumination light emitted from a light emitting portion of the light guide plate 113.
Also, whether the circuit substrate 102 is fixedly attached to the part of the light emitting surface 103b located toward the light entrance end surface 103a (refer to FIG. 7A) or to the slope surface 113b (refer to FIG. 7B), light, which is incident on the part of the light emitting surface 103b located toward the light entrance end surface 103a or incident on the slope surface 113b, is absorbed by the circuit substrate 102 when it is reflected there, which also lowers the brightness of illumination light.